Your post is somewhat difficult to unpack. Ultimately, you cannot get the charges dismissed. Only the prosecutor or your boyfriend's attorney may do that.
Once the police are involved, the decision to charge the crime is no longer yours. You may be asked for your input, but ultimately, the police forward the information to the prosecutor, and the prosecutor decides how to proceed.
In this case, we are beyond charging. I cannot tell you whether the deferred prosecution agreement is one where the charges are dismissed at the end or expunction is granted without more details. But, the victim witness personnel should be able to explain the terms of the deferred prosecution agreement to you.
A deferred prosecution should not be a "Plea of Guilty." Your boyfriend needs to consult a lawyer in your area to see if this case can 1) be removed from the diversion program and 2) if there is a chance of winning this case at trial; and 3) if it is best for him to stay on this diversion program.
I am assuming your boyfriend is charged with some form of domestic violence. Victim intimidation is a common charge that goes along with domestic violence. Unfortunately, prosecutors rarely listen to the victim's wishes in the prosecution of domestic violence. Many times the victims recant their testimony and attempt to assist the defense, feeling that her boyfriend or husband should not be punished. Many times, prosecutors do not listen to these pleas.
There is a huge difference in domestic violence cases between hiring private counsel and being represented by the public defender's office. Generally, private counsel will take the time to sit down and interview the victim and other witnesses. If the victim witness recants or is uncooperative with the prosecution, a private attorney can use that to the defendant's advantage.
Deferred prosecutions or stayed adjudications are different in different states, but it is probably not favorable for him to try to get out of this bargain. You should seek the advice of a local criminal defense attorney. I cannot stress enough the difference private counsel makes in these matters.